The present invention relates generally to fuel injectors, and more particularly, to fuel conduits for fuel injectors and the injectors for gas turbine engine combustors.
Fuel injectors, such as in gas turbine engines, direct pressurized fuel from a manifold to one or more combustion chambers. Fuel injectors also prepare the fuel for mixing with air prior to combustion. Each injector typically has an inlet fitting connected to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel conduit or passage (e.g., a tube, pipe, or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle. The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into the combustor chamber. An air cavity within the stem provides thermal insulation for the fuel conduit. A fuel conduit is needed that can be attached to a valve housing and to the nozzle. The fuel conduit should be tolerant of low cycle fatigue (LCF) stresses caused by stretching of the stem which houses the conduit and which undergoes thermal growth more than the cold conduit. The attachment of the conduit to the valve housing should be a reliable joint which doesn""t leak during engine operation. Fuel leaking into the hot air cavity can cause detonations and catastrophic over pressures.
A fuel injector typically includes one or more heat shields surrounding the portion of the stem and nozzle exposed to the heat of the combustion chamber. The heat shields are used because of the high temperature within the combustion chamber during operation and after shut-down, and prevent the fuel from breaking down into solid deposits (i.e., xe2x80x9ccokingxe2x80x9d) which occurs when the wetted walls in a fuel passage exceed a maximum temperature (approximately 400xc2x0 F. (200xc2x0 C.) for typical jet fuel). The coke in the fuel nozzle can build up and restrict fuel flow through the fuel nozzle rendering the nozzle inefficient or unusable. One such heat shield assembly is shown in U.S. Pat. No. 5,598,696 and includes a pair of U-shaped heat shield members secured together to form an enclosure for the stem portion of the fuel injector. At least one flexible clip member secures the heat shield members to the injector at about the midpoint of the injector stem. The upper end of the heat shield is sized to tightly receive an enlarged neck of the injector to prevent combustion gas from flowing between the heat shield members and the stem. The clip member thermally isolates the heat shield members from the injector stem. The flexibility of the clip member permits thermal expansion between the heat shield members and the stem during thermal cycling, while minimizing the mechanical stresses at the attachment points.
Another stem and heat shield assembly is shown in U.S. Pat. No. 6,076,356 disclosing a fuel tube completely enclosed in the injector stem such that a stagnant air gap is provided around the tube. The fuel tube is fixedly attached at its inlet end and its outlet end to the inlet fitting nozzle, respectively, and includes a coiled or convoluted portion which absorbs the mechanical stresses generated by differences in thermal expansion of the internal nozzle component parts and the external nozzle component parts during combustion and shut-down. Many fuel tubes also require secondary seals (such as elastomeric seals) and/or sliding surfaces to properly seal the heat shield to the fuel tube during the extreme operating conditions occurring during thermal cycling. Such heat shield assemblies as described above require a number of components, and additional manufacturing and assembly steps, which can increase the overall cost of the injector, both in terms of original purchase as well as a continuing maintenance. In addition, the heat shield assemblies can take up valuable space in and around the combustion chamber, block air flow to the combustor, and add weight to the engine. This can all be undesirable with current industry demands requiring reduced cost, smaller injector size (xe2x80x9cenvelopexe2x80x9d) and reduced weight for more efficient operation. Because of limited fuel pressure availability and a wide range of required fuel flow, many fuel injectors include pilot and main nozzles, with only the pilot nozzles being used during start-up, and both nozzles being used during higher power operation. The flow to the main nozzles is reduced or stopped during start-up and lower power operation. Such injectors can be more efficient and cleaner-burning than single nozzle fuel injectors, as the fuel flow can be more accurately controlled and the fuel spray more accurately directed for the particular combustor requirement. The pilot and main nozzles can be contained within the same nozzle stem assembly or can be supported in separate nozzle assemblies. Dual nozzle fuel injectors can also be constructed to allow further control of the fuel for dual combustors, providing even greater fuel efficiency reduction of harmful emissions.
A typical technique for routing fuel through the stem portion of the fuel injector is to provide a fuel conduit having concentric passages within the stem, with the fuel being routed separately through different passages. The fuel is then directed through passages and/or annular channels in the nozzle portion of the injector to the spray orifice(s). U.S. Pat. No. 5,413,178, for example, discloses concentric passages where the pilot fuel stream is routed down and back along the main nozzle for cooling purposes. This can also require a number of components, and additional manufacturing and assembly steps, which can all be contrary to desirable cost and weight reduction and small injector envelope.
U.S. Pat. No. 6,321,541 addresses these concerns and drawbacks with a fuel injector that includes an inlet fitting, a stem connected at one end to the inlet fitting, and one or more nozzle assemblies connected to the other end of the stem and supported at or within the combustion chamber of the engine. A fuel conduit in the form of a single elongated laminated feed strip extends through the stem to the nozzle assemblies to supply fuel from the inlet fitting to the nozzle(s) in the nozzle assemblies. An upstream end of the feed strip is directly attached (such as by brazing or welding) to the inlet fitting without additional sealing components (such as elastomeric seals). A downstream end of the feed strip is connected in a unitary (one piece) manner to the nozzle. The single feed strip has convolutions along its length to provide increased relative displacement flexibility along the axis of the stem and reduce stresses caused by differential thermal expansion due to the extreme temperatures in the combustion chamber. This reduces or eliminates a need for additional heat shielding of the stem portion of the injector.
The laminate feed strip and nozzle are formed from a plurality of plates. Each plate includes an elongated, feed strip portion and a unitary head (nozzle) portion, substantially perpendicular to the feed strip portion. Fuel passages and openings in the plates are formed by selectively etching the surfaces of the plates. The plates are then arranged in surface-to-surface contact with each other and fixed together such as by brazing or diffusion bonding, to form an integral structure. Selectively etching the plates allows multiple fuel circuits, single or multiple nozzle assemblies and cooling circuits to be easily provided in the injector. The etching process also allows multiple fuel paths and cooling circuits to be created in a relatively small cross-section, thereby, reducing the size of the injector.
The feed strip portion of the plate assembly is mechanically formed such as by bending to provide the convoluted form. In one embodiment the plates all have a T-shape in plan view. In this form, the head portions of the plate assembly can be mechanically formed into a cylinder having an annular cross-section, or other appropriate shape. The ends of the head can be spaced apart from one another, or can be brought together and joined, such as by brazing or welding. Spray orifices are provided on the radially outer surface, radially inner surface and/or ends of the cylindrical nozzle to direct fuel radially outward, radially inward and/or axially from the nozzle.
It is desirable to have a fuel conduit that is more flexible, has less bending stress, and is therefore less susceptible to low cycle fatigue than a single feed strip design. For example, individual strips of a dual strip design, each having thickness xc2xd that of a single strip design will have about xe2x85x9 the stiffness of a single strip and therefore significantly reduced LCF stresses for the same thermal growth differential. It is also desirable to have inherent damping to reduce vibratory stresses. The dual strip design has inherent damping and is therefore less susceptible to high cycle fatigue than the single feed strip design. It is also desirable to have a feed strip with convolutions along its length to provide increased relative displacement flexibility along the axis of the stem and reduce stresses caused by differential thermal expansion due to the extreme temperatures in the combustion chamber. It is also desirable to have a feed strip that provides a smaller envelope for the heat shield which, in turn, has a small circumferential width in the flow and lower drag and associated flow losses making for a more aerodynamically efficient design.
A fuel injector conduit has at least two generally parallel feed strips that are not bonded together along substantially their entire lengths. Each of the feed strips is constructed from a single bonded together pair of lengthwise extending plates and each plate has a single row of widthwise spaced apart and lengthwise extending parallel grooves. The plates in each of the strips are bonded together such that opposing grooves in each of the plates are aligned forming internal fuel flow passages through the length of the strip from an inlet end to an outlet end of the strip. The inlet ends are spaced apart from each other. Each of the feed strips has one or more convolutions along a length of the strips and the feed strips are not bonded together along the length of the strips that include the convolutions. The feed strips have fuel inlet holes in the inlet ends and are connected to the internal fuel flow passages. Each of the internal fuel flow passages is connected to at least one of the inlet holes. The convolutions of the feed strips may be spaced apart from each other or may be in contact with each other.
An exemplary embodiment of the fuel injector includes an upper housing, a hollow stem depending from the housing, at least one fuel nozzle assembly supported by the stem, and the fuel injector conduit extending between the housing through the stem to the nozzle assembly. The injector includes a fitting fluidly connecting all of the outlet ends to a single nozzle fuel conduit of the fuel nozzle assembly. The nozzle is constructed from a multi-layered arrangement of plates with internal fuel flow circuits located between the plates. Multiple spray orifices are fluidly connected to the internal fuel flow passages in the feed strips by the internal flow circuits. The injector has at least one fuel dispensing nozzle which may have a cylindrical configuration. The fuel dispensing nozzle may be a main nozzle and the injector further includes a pilot nozzle disposed centrally within the fuel nozzle. The pilot nozzle is fluidly connected to at least one of the internal flow circuits.
The present invention provides a fuel conduit that is more flexible, which reduces bending stress, and has inherent damping, which reduces vibratory stresses, and therefore is less susceptible to both low cycle and high cycle fatigue than a single feed strip design. The feed strip of the present invention has improved relative displacement flexibility along the axis of the stem and improved reduction of stresses caused by differential thermal expansion due to the extreme temperatures in the combustion chamber. The present invention provides for a fuel conduit that allows the use of a smaller envelope for the heat shield which, in turn, has a small circumferential width in the flow and, therefore, lowers drag and associated flow losses making for a more aerodynamically efficient design.